While the underlying goal of cancer therapy is to destroy the cancer while avoiding excessive damage to the normal organs of the body, their toxic effects to the body limit present treatments such as chemotherapy and radiation. As such, the maximal tolerable dosage of such therapies is often inadequate to eradicate the tumor. Newer treatment strategies have focused upon identifying antineoplastic agents that can distinguish normal cells from their cancerous counterparts. Oncolytic viruses replicate, spread and selectively destroy cancerous tissue, but are attenuated and do not harm normal cells. In addition to direct oncolysis, an immune-mediated component contributes to oncolytic virus efficacy in immune-competent mice (i.e., oncolytic viruses have a tumor-vaccination effect mediated at least in part through an anti-tumor CD8+ T cell response). Using immune-competent mice with syngeneic, bilateral subcutaneous (s.c.) tumors, previous studies established that treatment of one tumor with oncolytic virus (HSV-1) induced regression of the treated and untreated contralateral tumor (see Toda M, et al. “Herpes simplex virus as an in situ cancer vaccine for the induction of specific anti-tumor immunity.” Hum Gene Ther 1999; 10:385-93). While treated and untreated tumors both regressed, oncolytic virus was only detected in the treated tumor. Furthermore, regression of the uninjected, contralateral tumor resulted from an anti-tumor CD8+ T-cell response.
Several different oncolytic herpes simplex virus type 1 (HSV-1) strains have proven to be safe in phase I human clinical trials. See Aghi & Martuza, Oncogene (2005) 24:7802-7816. Viral genetic analysis has established that HSV-1 can be effectively neuro-attenuated by deleting the γ134.5 neuropathogenesis genes. Chou et. al., Science (1990) 250:1262-1266. The cellular interferon-induced eIF2α kinase PKR, a major innate host defense component, phosphorylates the critical host cell translation initiation factor eIF2α in response to viral infection. Phosphorylated eIF2α blocks translation initiation thereby precluding the manufacturing of viral polypeptides and progeny. The γ134.5 gene encodes a regulatory subunit of the cellular protein phosphatase 1 and directs dephosphorylation of eIF2α which results in the production of viral proteins and progeny. Chou et al., Proc. Natl. Acad. Sci. USA (1995) 92:10516-10520; He et. al., Proc. Natl. Acad. Sci. USA (1997) 94:843-848. While γ134.5-deficient (Δ34.5) viruses are sufficiently attenuated and safe (see, U.S. Pat. No. 7,981,669 by Coffin et al.), their anti-tumor efficacy in animal models is severely limited by their constrained ability to replicate in many types of cancer cells.
Failure of these Δ34.5 strains to propagate an infection throughout the tumor mass allows the cancer to simply regrow. See Mohr, Oncogene (2005) 24:7697-7709. The HSV-1 Us11 gene has been shown to encode a function expressed very late in the viral growth cycle that antagonizes PKR and innate host defenses. Viruses engineered to express Us11 very early following infection (termed “immediate-early” of “IE”) allow Δ34.5 mutant viruses to grow efficiently. Remarkably, Δ34.5 viruses that express IE Us11 (Δ34.5 IE Us11) remain just as neuro-attenauted as the parental Δ34.5 strains, yet they replicate in and efficiently destroy cancer cells, making them ideal oncolytic virus candidates. Mohr et. al., J. Virol. (2001) 75:5189-5196. In studies using independently constructed viruses in different tumor models, engineering a Δ34.5 mutant derivative to express IE Us11 resulted in a dramatic improvement in the ability of the virus to inhibit tumor growth. Taneja et. al., Proc. Natl. Acad. Sci. USA (2001) 98:8804-8808; Todo et. al., Proc. Natl. Acad. Sci. USA (2001) 98:6396-6401; and Liu et al., Gene Therapy (2003) 1):292-303.
However, the above-described Δ34.5 IEUs11 oncolytic strains have a major drawback, as engineering IE Us11 expression inactivates the neighboring Us12 gene, which encodes an important immunomodulatory polypeptide, ICP47, involved in blocking antigen presentation by inhibiting the transporter associated with antigen presentation (TAP) 1/2. Mohr et al., J. Virol. (1996) 75:5189-5196; Todo et al., Proc. Natl. Acad. Sci. USA (2001) 98:6396-6401; Liu et al., Gene Therapy (2003) 10:292-303. Since the Us12 gene product acts to inhibit antigen presentation, its absence results in increased clearance of infected cells by the acquired immune response. Goldsmith et al., J. Exp. Med. (1998) 187:341-348. Thus, Us12 is likely required to ensure that the HSV-1 oncolytic virus is not prematurely cleared before it has a chance to spread through the tumor tissue and complete its task of tumor eradication. This is especially important given the prevalence of HSV-1 and HSV-1-specific immunity (e.g., seropositivity) in the general population. Indeed, recently published studies indicate that evasion of CD8+ T cells is critical for superinfection by a herpesvirus. Hansen et al., Science (2010) 328:102-106. Although it is understood that Us12 prevents cytolytic T-cell recognition of infected cancer cells, it does not interfere with presentation of tumor antigens on the surface of uninfected cells or, after infection begins, down-regulate existing cell surface complexes displaying tumor antigens. Hence, expression of Us12 immunomodulatory activity enhances viral spread and oncolysis but does not diminish the overall immune response and/or potential for creating a tumor vaccination effect.
Δ34.5 IEUs11 HSV variants having intact Us12 were described in U.S. Pat. No. 7,731,952 by Mohr et al. While those Δ34.5 IEUs11 HSV variants expressed Us12, it is not possible to test those variants in murine models of, e.g., cancer, using immune-competent mice, because Us12 cannot inhibit murine TAP, leading to the premature clearance of virus-infected cells, as discussed above.
Animal models are often instructive in understanding human diseases, and it would be useful to be able to test Δ34.5 IEUs11 HSV variants in such models, especially ones that use immune-competent mice in order to more closely represent human diseases, such as cancer, in which most patients are immune-competent and may also have anti-HSV specific memory T cells. Hence, there remains a need in the art for oncolytic viruses that evade CD8+ T cells and/or avoid premature clearance by the immune system, particularly ones that can be tested in immune-competent murine and human models.